Abundances of r -Process Elements in Stars J. E. Lawler, C. Sneden, J. J. Cowan, & E. A. Den Hartog, Univ. of Wisconsin – Madison, Univ. of Texas – Austin, Univ. of Okalahoma – Norman (A perspective from a laboratory spectroscopist. Sharp line spectroscopy & the possibility of line spectroscopy in kilonova)
Fraunhoffer Lines In 1802 William Hyde Wollaston noted dark features in the Sun’s spectrum. In 1814 Joseph von Fraunhoffer also found these and launched a careful study of the features. Wavelengths were measured using prisms, most prominent features were “named” A through K, less prominent were given other names,…. Wikipedia
Fraunhoffer Lines in the Sun & other Stars are from the temperature gradient. • For the Sun, T = 5778 K at surface • Much like a black body but T increases with depth • Deeper hotter layers provide a continuum for absorption features from outer cooler layers
Continuum from hotter interior yields absorption lines from cooler layer near surface Real photospheric models do not have step boundaries. Temperature gradient is modeled using radiation transport equation typically with LTE/1D approximations.
Elements (Z > 30) are made by neutron ( n -)capture. Some elements & isotopes are made primarily by the slow ( s -)process, others by the rapid ( r -)process. H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr NbMo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Rf Db Sg Bh Hs Mt UunUuuUub La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu Ac Th Pa U Np PuAmCmBk Cf EsFmMdNo Lr
n -Capture element definitions • s -process: all β -decays can occur between successive n -captures - Site: AGB (Red Giant) Stars (proof: Tc short lived 200 kyear) in spectra • r -process: rapid, short-lived neutron blast temporarily overwhelming β - decay rates …. n (eutron)-star (NS) mergers are a site of r - process based on electromagnetic follow up studies of GW-170917 • r - or s -process element: ones whose origin in solar-system material was dominated by one or the other process
Nucleosynthesis by n (eutron) – capture Some Milestones • Paul & Merrill 1952, Tc (200 kyr lifetime) in Red Giant (AGB) Star…the s(low)- process n- capture site…stellar wind • Hoyle’s 1954 paper • B2FH Rev. Mod. Phys. 29, 547 (1957) • SN 1987A …first nearby SN since Kepler…proton decay experiments detect neutrino burst… • a-LIGO + VIRGO in 2017 find a n -star merger & teams study the electromagnetic flash
r- process is still tough to model • Important nuclei are far from stability • Facility for Rare Isotope Beams (FRIB) at MSU and facilities in Europe and Japan will produce the needed nuclear data • Old metal poor (MP) stars enable us to trace nucleo-synthesis • Big Telescopes, UV access with HST, & better Lab Astro
Key Question? Is there any hope of seeing freshly made r - process elements via lines in absorption or emission? How about emission after the cloud has expanded and cooled? Harriet Dinerstein at UT – Austin has seen forbidden emission lines of n- capture elements in multiple nebulae. The answer for the near term is NO Sharp Lines but …… ! The NS fireball is throwing out material at several tenths of c and a wide range of directions.
Sharp Line Spectroscopy • Sharp line spectroscopy has greatly improved • Review in the following slides is on Metal Poor (MP) stars & their relevance to the r -process • After the review we will consider some possibilities for sharp line spectroscopy on kilonova.
MP stars have simpler spectra & are sometimes rich in n- capture elements
Isotopes built by n -capture syntheses The valley of -stability Rolfs & Rodney (1988)
The s -process can now be modeled • Nuclei of interest are either stable or slightly radioactive • Many or most needed nuclear data have been measured • Model s -process abundances can be subtracted from the total Solar System elemental abundance to determine the r - process Solar System abundance
Site of r- process? Type II (core collapse) Supernovae are the leading candidate at this time. Stellar mass > 9 Solar Mass, Fe core > 1.44 Solar mass. The expanding remnant of SN 1987A, a Type II-P supernova in the Large Magellanic Cloud, NASA image.
From Woosley & Janka 2005
Site of r- process? • NS mergers are surely a site of r- process nucleosynthesis. • There is only a few 1000 Solar Masses or less of n- rich elements in the entire Galaxy. • Short GRBs are likely from NS mergers • There is still Lab Astro to be done.
r -PROCESS IN NEUTRON STAR MERGERS C. Freiburghaus, S. Rosswog, and F.-K. Thielemann ApJ 525:L121(1999) Credit to Thielmann’s group for early work on n -star mergers does not detract from the many contributions of people here.
Fernández et al. 2017 BH + NS merger
In the decades after Fraunhoffer absorption lines were matched to atoms and ions • Wavelength measurements improved steadily • Large grating spectrographs (Rowland Circle) were used to achieve ppm accuracy in the first half of the 20 th Century • Wavelength measurements could be improved to 10 ppb by late 1970s w FTS instruments. • Optical frequency combs can now achieve better than 0.0000001 ppb (one line at a time)
What was left to work on circa 1980 in spectroscopy? • Einstein A coefficients are essential to quantitative spectroscopy • No really good (fast, accurate, v broadly applicable) measurement technique was available until tunable lasers • Organic dye laser 1966 (P. Sorokin & F. Schafer et al.) provided broad tunability • Dye lasers needed improvements but by mid 1970s they were ready
Pulsed Dye Lasers by mid 1970s • Optical bandwidth of a few GHz ~ Doppler width of atom & ion lines • Pulse duration of a few nsec, v low Q cavity yields v abrupt pulse termination • Rep Rate 10 – 100 Hz well matched to fast data handling system • Tunability 200 nm – 800 nm some non-linear crystals needed • Dye lasers had been mastered by multiple groups
UW Lab Astro developed the atom/ion beam source 1980 - 81 • It works well with all metallic and most non- metallic elements • It is highly reliable, down time < 1% • It delivers 10 14 atoms/(sec sr) • The beam is rich in metastable atoms and ions, one can use levels 4 eV above the ground level as a lower level for LIF • Time Resolved Laser Induced Fluorescence (TR LIF) yielded radiative lifetimes ( tau’ s) accurate and precise to ~ a few % • Many tau ’ s can be measured per day
u 1/ tau u = Sum A ui A u4 A u3 A u2 4 A u1 3 2 BF uk = A uk / Sum A ui = A uk tau u 1
Search for possible systematic errors • Radiation trapping? Vary the beam density • Collisional quenching? Throttle the pump • Zeeman quantum beats? B = 0 (~ 10 milliGauss) for short lifetimes, B ~ 25 Gauss for long lifetimes • Ultimate end-to-end test: Periodic re-measurement of benchmark lifetimes in He, Be,….
Comparison of Sm II lifetimes from UWO vs UW
Clearly, LIF experiments can provide accurate, absolute radiative lifetimes. Ab-initio theory provide good branching ratios in simple spectra, experiments provide good branching rations in complex spectra.
Advantages of an FTS: Kitt Peak ( James Brault ), NIST, Lund • Very high spectral resolving power • Excellent absolute wavenumber accuracy • Extremely broad spectral coverage • Very high data collection rates • Insensitive to source intensity drifts • Large etendue • Ward Whaling (Caltech) relative radiometric calibration of FTS
Comparison of Sm II A coefficients UWO vs UW
Comparison of Oxford to UW log(gf)s for Ti I
Comparison of Oxford to UW log(gf)s for Ni I
Attention must be paid to hyperfine, isotopic structures: typical La II lines Solar photosphere: green lines Hyperfine components: red sticks No isotopic worries; only 139 La Log (X) = log(N X /N H ) + 12 Lawler et al. 2001
Classic hfs Flag Pattern of UV Ho II line
New Rare Earth Element Abundance Distribution for the Sun and Five r- Process-Rich Very Metal-Poor Stars C. Sneden et al. ApJS 182:80 (2009) Tightly define r -process abundance pattern will constrain future modeling efforts. (Tens of person-years work underlie this plot.)
Key Questions? Is the r -process abundance pattern the same for NS mergers and core- collapse SNe? Is the r -process abundance pattern simply determined by fission recycling and/or related nuclear physics?
THE RISE OF THE s -PROCESS IN THE GALAXY J. Simmerer et al. ApJ 617:1091 (2004)
Key Questions? Clearly the r -process turned on abruptly when the Galaxy & Universe were young. Is it possible to explain most or all r - process material using NS mergers? Possible but better statistics are needed. How is it possible to make lots of NS binaries in tight orbits from the first generation of stars? Is Inhomogenity the explanation?
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